1. Field of the Invention
The present invention relates to an optical compensation sheet and a polarizing plate, which are used for a TN-mode liquid crystal display device. The present invention also relates to a TN-mode liquid crystal display device having the optical compensation sheet or polarizing plate.
2. Description of Related Art
Various optical compensation sheets including a transparent support having thereon an optically anisotropic layer formed of a liquid crystal composition have been proposed as an optical compensation sheet for a TN-mode liquid crystal display device,
For example, JP-A-8-50206 has proposed an optical compensation sheet including a transparent support having provided thereon an optically anisotropic layer, in which the optically anisotropic layer is a layer having a negative birefringence and including a compound having a discotic structural unit, the disc plane of the discotic structural unit is inclined with respect to the transparent support plane, and the angle made by the disc plane of the discotic structural unit and the transparent support plane is being changed in the depth direction of the optically arisotropic layer.
The TN-mode liquid crystal display device using an optical compensation film including an optically anisotropic layer on a transparent support can realize a high contrast and a wide contrast-viewing angle, but in view of production, a step of coating the optically anisotropic layer on the transparent support is included and this involves a high cost and gives rise to increase in the unit price of a liquid crystal panel. Therefore, there has been proposed a TN-mode liquid crystal display device using an optical compensation layer obtained by imparting optical characteristics to the transparent support without forming an expensive layer such as a layer formed of a liquid crystal layer composition as described above (JP-A-2002-22942).
However, the TN-mode liquid crystal display device using an optical compensation layer of a transparent support having optical characteristics is significantly decreased in the display performance as compared with a TN-mode liquid crystal display device using an optical compensation film including an optically anisotropic layer. In particular, tone reversal in the downward direction of the panel, or color tinting in the oblique direction at the display in black or halftone, which are peculiar to the TN mode, is a serious problem. As for means for improving the tone reversal, a technique of using a surface diffusing film on the viewing side of the liquid crystal panel has been proposed (JP-A-2003-279736), but means for improving the change in color tint has not been studied.